How can I attack another player's castle?

First, leave your city to go to the map.Select the castle you want to attack and select View from that castle's menu.Once you are inside the target castle, you can click the Scout button at the bottom center of the screen to find out what troops are defending the castle.Then, click the Attack button at the bottom center of the screen.Click the images of your troops to place the amount of soldiers you would like into the available columns. The cost to place the troops you've selected is shown on the left side of the screen.Once your troops are in the columns, click Attack to send your army on the campaign.